1. Field of the Invention
The invention concerns a process for regenerating an electroless copper plating bath containing a complexing agent, such as ethylenediamine tetraacetic acid or the like. The invention also concerns apparatus for implementing that process.
2. Description of the Related Art
Chemical copper plating baths, i.e., copper plating baths operating without connection to an external current source, are used, for instance, to coat plastic surfaces, to uniformly coat components of complex geometry, in particular to produce printed circuits by the semiadditive or the fully additive method. A feature in common to all chemical baths is that the stock of metal used for coating has to be introduced into the bath in a dissolved form. However, to obtain a passable deposition, the concentration of free metal has to be greatly limited. For that purpose, complexing agents are used which mask the metal cation and which, to maintain complexation equilibrium, release the metal cation in small quantities for the coating reaction. To limit the concentration of free metal cations as necessary, complexing agents are often added to the bath in quantities which are several times higher than those actually required. Ethylenediamine tetraacetic acid (EDTA) is most frequently used as a complexing agent.
To ensure that the copper film deposited by the electroless process has excellent physical properties, compared with electrolessly deposited films produced by the subtractive method that merely serve as a conductive thin film for a throughhole and on which copper is electrolytically deposited, it is essential that the composition of the electroless copper plating bath be controlled as accurately as possible so that its concentration is highly uniform and the formation of by-products is minimized. The latter is particularly essential in conjunction with the recovery of the complexing agent, preferably of the ethylenediamine tetraacetic acid existing in the electroless copper plating bath in high concentrations.
According to one known process, the copper plating bath containing the complexing agent is taken from the plating tank in full or in part, the copper content of the bath is reduced by precipitating the copper as metal copper or copper oxide or by using electrolysis, and by subsequently precipitating the complexing agent by acidification. The complexing agent thus recovered is returned to the anodic portion of a cell comprising a copper anode separated by an ion exchange membrane from the cathodic portion comprising the cathode. Then, DC current is applied to both electrodes, and the solution is fed back from the anodic portion of the cell to the electroless copper plating bath. In conjunction with this process, the effect of the conditions of electrolysis for reducing the copper content and how such conditions would influence the purity of the recovered complexing agent, preferably the EDTA, is unknown.
In a known process for decontaminating chemical plating baths, a heavy metal is removed from solution by selectively operating ion exchangers and the residual solution containing the complexing agent is processed further. However, this process is only suitable for separating metals from solutions containing complexing agents, the stability constant of which is less than that of the exchange resin. This does not apply to EDTA.